Pigmentation of non-aqueous inks and coatings is facilitated by various forms of milling the pigment into the carrier vehicle. Some pigments are very difficult to disperse and stabilize into these systems. Modification of the pigment surface to promote the formation of effective adsorption layers of polymeric molecules (resins, dispersants, and surfactants) has to be found to be a solution for producing stable pigment dispersions with satisfactory properties at a reasonable cost. It is not unusual for organic pigments, especially polycyclic pigments, to have a surface with low polarity (lack of functional groups, strong electric charges, active centers, and the like) (R. Sappok, J. Oil Col. Chem. Assoc., 61 (1978), 299-308). In many cases, the low polarity may be explained by the morphology of the primary pigment crystals, and intra- and inter-molecular hydrogen bonding (W. Herbst, K. Hunger, “Industrial Organic Pigments”, VCH, 1997). As a result, the adsorption energy for a wide array of polymers and surfactants is low, and may be characterized as a physical adsorption. This type of adsorption is reversible, and it may not provide an efficient barrier to flocculation, due to the desorption process.
One of the oldest and popular methods for surface modification is the use of synergists, which are derivatives of organic pigments with a strong affinity for the pigment surface. One of the earliest original studies was carried out with phthalocyanine pigments (W. Black, F. T. Hesselink, A. Topham, Kolloid-Zeitschrift and Zeitschrift für Polymere, Bd.213, Heft 1-2, (1966), 150-156). As soon as the mechanism of stabilization of the dispersions was understood, the approach was extended successfully for other classes of organic pigments (J. Schroeder, Progress in Organic Coatings, 16 (1988), 3-17; and U.S. Pat. Nos. 3,996,059; and 4,057,436).
The synergists could be soluble or insoluble in application media. The soluble compounds are similar to dyes. Less soluble synergists behave more like pigments. The mechanism of surface modification with soluble or partially soluble compounds can be described in terms of adsorption from the liquid phase. The adsorption energy, the thickness, and the structure of the adsorption layers may be determined from the isotherm of adsorption and other experimental data. The mechanism of surface modification with insoluble synergists is not quite clear. It is believed that the synergists make up solid solutions with pigment crystals, or at least partially penetrate and modify their crystal lattice. The insoluble synergists often demonstrate the ability to stabilize pigment particles against re-crystallization in solvent based dispersions and plastics, especially at elevated temperatures (U.S. Pat. Nos. 4,141,904; 4,981,888; 5,264,032). They are also known to be able to direct and control the growth, crystal phase, and morphology of pigment particles (U.S. Pat. Nos. 6,225,472; 6,264,733).
In general, a synergist consists of a moiety of the pigment, one or more functional groups, connecting groups, and other substituting groups. A wide variety of synergists representing various classes of chromophores and functional groups, are described in relevant literature. Examples of chromophores are: phthalocyanines, quinacridones, quinacridonequinones, anthraquinones, perylenes, azo, azomethines, benzimidazolones, perinones, diketopyrrolopyrroles, isoindolines, isoindolinones, iminoisoindolines, iminoisoindolinones, flavanthrones, dioxazines, indanthrones, anthrapyrimidines, quinophthalones, indigo, thioindigo, isoviolanthrones, and pyranthrones. Functional and connecting groups are represented by sulfonic acids (U.S. Pat. Nos. 4,726,847; 5,296,033; 5,296,034; 6,083,315); metal salts of sulfonic acids (U.S. Pat. Nos. 3,386,843; 5,264,034; 5,275,653; 5,989,333; 6,251,553; 6,793,727); salts of sulfonic acid with primary (U.S. Pat. Nos. 6,471,764; 6,926,768), secondary (U.S. Pat. No. 6,152,968), tertiary (U.S. Pat. No. 5,472,494), and quaternary (U.S. Pat. Nos. 3,754,958; 4,057,436; 5,271,759; 6,641,655; 6,648,954) amines; sulfonamides (U.S. Pat. Nos. 4,310,359; 5,427,616; 6,066,203; 6,123,763; 6,454,845; 6,827,775; 7,045,637; 7,045,638), and their mixtures with sulfonic acids (U.S. Pat. Nos. 4,350,632; 4,952,688; 5,779,783); substituted benzamidomethyl and structurally related phthalimidomethyl and sulfobenzimidomethyl derivatives (U.S. Pat. Nos. 3,635,981; 4,197,404; 4,256,507; 4,439,240; 4,455,173; 4,478,968; 4,541,872; 4,844,742; 4,895,949; 5,194,088; 5,264,032; 5,286,863; 5,424,429; 5,453,151; 5,457,203); pyrazolylaminomethyl (U.S. Pat. No. 5,334,727); melaminomethyl (U.S. Pat. No. 6,225,472); barbituratomethyl (U.S. Pat. No. 6,225,472); arylmethyl (U.S. Pat. No. 6,264,733); alkyl amines (U.S. Pat. No. 5,718,754); carboxylic acids (U.S. Pat. No. 6,689,525); salts (U.S. Pat. Nos. 5,296,033; 5,296,034; 5,989,333; 6,918,958), amides (U.S. Pat. Nos. 4,104,275; 4,310,359; 6,123,761; 6,284,031; 6,689,525), and esters (U.S. Pat. No. 6,689,525) of carboxylic acids; carbonyl (U.S. Pat. No. 6,689,525); amidomethyl (U.S. Pat. Nos. 5,250,111; 5,476,544; 5,711,800; 6,123,763); alkylaminomethyl (U.S. Pat. Nos. 4,039,346; 4,104,276; 4,920,217; 5,427,616; 6,225,472); arylalkyloxy (U.S. Pat. Nos. 5,935,272; 5,998,621; 6,726,755); phenylthio and phenylamino (U.S. Pat. No. 5,516,899); and azomethine (U.S. Pat. No. 4,946,508). Numerous combinations of aforementioned groups are utilized as synergists as well (U.S. Pat. Nos. 4,888,422; 5,466,807; 5,472,494; 5,516,899; 5,958,129; 6,406,533; 6,918,958; 7,156,912). The applications of various classes of dyes as synergists are also known in the art (U.S. Pat. Nos. 6,264,733; 6,406,533; 7,074,267; U.S. Pat. App. 2005/0022695).
The synergists are used in all traditional applications such inks, coatings, plastics as well as in other areas like electrophotographic toners and developers, optical filters, ink jet systems, and others.
Another known method of surface modification consists of introducing functional groups during the course of synthesis of pigments by the partial replacement of intermediates with analogous intermediates carrying active groups. This technique of chemical modification is well-known for azo (U.S. Pat. Nos. 4,643,255; 5,021,090), phthalocyanine (U.S. Pat. No. 3,006,921), quinacridone (U.S. Pat. Nos. 5,755,873; 6,284,890; 6,494,948), and perylene (U.S. Pat. No. 6,692,562) pigments.
Finally, direct reaction on the surface of a pigment has been widely practiced as a way to modify surface properties. One of the earliest techniques was a treatment of azo pigments with primary amines, resulting in the formation of azomethine groups (Schiff's bases) (U.S. Pat. No. 4,468,255). Other reactions have been described as well: oxidation with sodium hypochlorite (U.S. Pat. Nos. 2,439,442; 3,347,632; 6,554,891), nitric acid (U.S. Pat. No. 3,023,118), and ozone (U.S. Pat. No. 6,852,156); sulfonation with sulfur trioxide (U.S. Pat. No. 6,821,334), or sulfur trioxide/pyridine complex (U.S. Pat. No. 5,928,419); Friedel-Crafts alkylation (U.S. Pat. No. 3,043,708), and acylation (U.S. Pat. App. 2006/0112853); amidation (U.S. Pat. No. 6,228,942); decomposition of diazonium salts (Gomberg-Bachmann reaction) (U.S. Pat. Nos. 5,554,739; 5,837,045; 6,221,932; 6,506,245 6,723,783; 6,780,389; 6,896,726); nucleophilic replacement of halogen (U.S. Pat. No. 6,641,653); and radical reaction of alkylarylhalides in the presence of transition metal salts (U.S. Pat. No. 6,852,158). The direct reactions on the pigment surface have been especially fruitful for carbon black pigments, due to their highly developed and reactive surface (U.S. Pat. Nos. 6,780,389; 6,852,156). Examples of other treated substrates include phthalocyanine (U.S. Pat. Nos. 6,641,653; 6,852,156), quinacridones (U.S. Pat. No. 6,852,156), azo (U.S. Pat. No. 6,833,026) and other organic pigments (U.S. Pat. Nos. 5,837,045; 6,821,334).
All three groups of surface modification methods have their advantages and disadvantages. For example, direct sulfonation or oxidation allows the introduction of active groups on the surface of a wide variety of pigments. However, the process requires the use of large amounts of solvents and toxic reagents, like sulfur oxides or ozone, making the process expensive and not environmentally friendly. Alternately, the method of diazonium salt decomposition is relatively simple, low-cost, and environmentally friendly. However, it has a serious limitation of not being efficient for all classes of organic pigments. With respect to chemically modified pigments, the functional groups do not always survive the harsh reaction conditions of pigment synthesis. This approach does not always provide the flexibility for design and production of various grades of pigments for different applications.
Thus, synergists play an important role for pigment surface modification. They can be prepared by simple and economical synthesis procedures, and offer the pigment manufacturer flexible means for producing high performance pigments and dispersions with improved coloristical, rheological, and stability properties.
In applications where dyes have been employed as coloring agents, organic pigments have been finding increasing utility in recent years for their excellent light fastness and resistance to solvents and bleed. These applications include, for example, inks for writing instruments, in which water or oil-soluble dyes have been used as coloring agents; and colorants for plastics, in which oil-soluble dyes have been used as highly transparent colorants. There are also increasing demand for organic pigments as coloring agents for LCD color filters, toners, and ink-jet inks (U.S. Pat. No. 6,726,762).
In ink-jet printing systems, a liquid ink is ejected from a nozzle towards a recording substrate using pressure, heat, or an electric field as the driving force. Ink-jet printing is excellent for printing variable information and can be used to print high-quality photographic images. In general, the ink vehicle can be aqueous or non-aqueous, and ink is referred to as aqueous or non-aqueous ink, accordingly. A more detailed classification of ink-jet systems takes in account the setting mechanism of ink (U.S. Pat. No. 7,141,104): absorption, penetration, and evaporation for water based inks; absorption and penetration for oil based inks; evaporation for solvent based inks; solidification at room temperature for hot melt inks that are liquid at ejection temperature; and polymerization for UV-curable inks.
The demand for outdoor use of ink-jet printed materials has been increasing. Applications include outdoor wrap, such as: vinyl advertising wrap covering trucks, billboards, posters, and signage. Therefore, non-aqueous pigmented inks are developed which can be printed directly on polymeric substrates, used outdoors without lamination, and have good weather resistance.
The requirements for ink-jet inks are quite rigorous, especially regarding the particle size distribution, rheological behavior, and colloidal stability. One of the major reasons for such demanding requirements for pigment dispersions is the size of the nozzles of printing heads. The diameter of the nozzle is normally in the range of 30-50 microns, so any type of instability in ink make the printing process troublesome, or totally impossible.
As it can be seen from the previous examples, the sulfonic group, in the form of a free acid, and more often as a salt with various metals or amines, is widely used for pigment surface modification. For example, salts of phthalocyanine sulfonic acid with Fe2+ and Fe3+ have been used for improving flow ability and stability of inks based on blue and green phthalocyanine pigments (U.S. Pat. No. 6,793,727).
A similar approach for enhancing the rheological properties (U.S. Pat. No. 7,077,898) is by treating carbon black with aluminum salts of sulfonic acids of quinacridone, perylene, indanthrone, dioxazine, and other polycyclic pigments. To improve the dispersibility of the additive itself, a texture-improving agent is incorporated during or after the synthesis, such as fatty acids, amides, amines, esters, alcohols, polyols, polyvinylalcohols, polyvinylpyrrolidones, oils, waxes, and resins.
The calcium salts of sulfoalkyl- and sulfoarylimides of perylene tetracarboxylic acid anhydride have been utilized for the improvement of the properties of perylene pigments such as color strength, dispersibility in high quality coatings and synthetic resins, resistance to flocculation, and rheological behavior (U.S. Pat. No. 5,264,034).
Sulfoarylmethylene derivatives of polycyclic pigments such as quinacridone, phthalocyanine, and diketopyrrolopyrrole are described in (U.S. Pat. No. 6,264,733). They are made by condensation of para-formaldehyde with a relevant pigment and a sulfonated aromatic compound, for example benzene or naphthalene sulfonic acid. The additives, in the form of the sulfonic free acid, metal, or ammonium salts are recommended as agents to control the growth and crystal phase of pigment particles. They are particularly useful when present during the synthesis of the pigment.
In order to improve the rheological, gloss, stability, and color properties of dioxazine pigments, they are treated with sulfo-containing dioxazine derivatives in the form of metal or ammonium salts, as well as mixtures of salts with alkylarylsulfates (U.S. Pat. No. 5,275,653).
Co-precipitation of sulfonic acids of polycyclic pigments with quaternary amines and calcium chloride is described in (U.S. Pat. Nos. 6,827,774 and 6,827,775). The synergist is claimed as a “rheology improver”.
Sulfonic acid groups can also be introduced onto the pigment surface through direct reactions with sulfur trioxide (U.S. Pat. No. 6,821,334) or decomposition of diazocompounds (U.S. Pat. No. 5,837,045).
All aforementioned patents offer some solutions for viscosity reduction and stabilization of pigment dispersions. However, they do not satisfy all stringent specifications that are required of pigments for non-aqueous inkjet inks; especially in respect to filterability of inks, storage stability, and high color strength.
Thus, there is a need for improved high performance pigments and their dispersions that provide non-aqueous ink-jet inks with excellent color, rheological, and stability properties.
In particular, quinacridone pigments such as PR122, PR202, PR207, PR209, PV19, pose problems with respect to flocculation, poor rheological behavior, and poor filterability in non-aqueous inks. PR122, which is process magenta for many applications including ink-jet inks, is especially problematic, as is noted in (U.S. Pat. No. 7,077,898). Other colors used for coloration of non-aqueous ink-jet inks also represent a substantial challenge for producers of non-aqueous ink-jet inks. These include phthalocyanine blue, carbon black, and different types of yellows.